Densification of Fibrous Structures by Resin Transfer Moulding for Making Thick Parts of Composite Material

ABSTRACT

A fiber structure for forming the reinforcement of a part to be made is placed in a mold ( 22 ) having at least one wall formed by a flexible membrane, and then a resin composition is injected into the mold, the composition having a volatile material content of less than 25% by weight and being at a temperature of a value such that its viscosity lies in the range 0.1 Pa·s to 0.3 Pa·s. The resin is polymerized in the mold placed in an enclosure ( 20 ) with temperature being raised progressively and with polymerization including at least a final stage of polymerization under pressure in order to obtain a composite material part presenting residual porosity of less than 11% by volume.

BACKGROUND OF THE INVENTION

The invention relates to making composite material parts with fiberreinforcement and a resin matrix by using a method of injecting resinunder pressure also referred to as resin transfer molding (RTM).

The field of application of the invention is more particularly that ofmaking thick parts out of composite material. As examples of such parts,mention can be made of the diverging portions of rocket engine nozzlessuch as those made of composite material having carbon fiberreinforcement and a phenolic resin matrix. Naturally, the invention isapplicable to fabricating a wide variety of parts, whether parts forrocket engines or airplane engines, or parts suitable for use moregenerally in the fields of aviation and space, or in other fields.

A technique commonly used for making thick parts of composite materialconsists in using a resin to pre-impregnate layers or strips of wovenfabric or other fiber textures, in draping or winding thepre-impregnated layers or strips on a shaper and a mandrel until adesired thickness is obtained, then covering the resulting blank in adelamination film, a resin drain fabric, and an elastomer membrane inorder to allow the resin to be polymerized in an autoclave and thusobtain a part having substantially the desired shape.

Such a method makes it possible to achieve technical results that aresatisfactory for certain potential applications, i.e. low residualporosity and a reinforcing fiber fraction that is quite high.Nevertheless, implementing that method industrially presents drawbacks:resin impregnation and polymerization within an autoclave after drapingor winding are implemented as a plurality of successive steps. Theoperation of impregnation by passing through baths implies usingsolvents and requires special treatment of effluents since they presentproblems in terms of the environment, hygiene, and safety.

The RTM method has also been known for a long time and it is inwidespread use, enabling the steps of impregnating a fiber reinforcementin a mold by injecting resin to be followed immediately bypolymerization in an autoclave without the impregnated fiberreinforcement being left in open air.

Nevertheless, if a conventional RTM method is implemented on fiberreinforcement of great thickness, it is difficult to obtain compositematerial parts presenting little residual porosity. In order to be ableto impregnate thick fiber reinforcement right through to the core it isnecessary for the resin to present low viscosity. Lowering viscosity byusing solvents, and using resins that give off volatile materials duringpolymerization, as applies in particular for phenolic resins, mean thata high level of residual porosity is present in the composite materialafter the resin has been polymerized. It is indeed possible to reduceporosity by repeating the impregnation and polymerization cycle severaltimes, but only with significantly increased durations and processingcosts.

OBJECT AND SUMMARY OF THE INVENTION

An object of the invention is to provide a method of making compositematerial parts with fiber reinforcement and a resin matrix, suitable forobtaining parts that are thick, with low porosity, and withoutpresenting the above-mentioned drawbacks of prior art methods usingpre-impregnated reinforcement or a conventional RTM process.

This object is achieved by a method of making a thick part of compositematerial having fiber reinforcement and a resin matrix, the methodcomprising the steps of:

-   -   providing a fiber structure that is to form the reinforcement of        the part to be made;    -   placing the fiber structure in a mold having at least one wall        that is formed by a flexible membrane;    -   injecting into the mold a resin composition having a volatile        material content of less than 25% by weight and at a temperature        of a value such that its viscosity lies in the range 0.1 pascal        seconds (Pa·s) to 0.3 Pa·s; and    -   polymerizing the resin in the mold placed in an enclosure with        temperature being raised progressively, the polymerization step        including at least a final stage of polymerization under        pressure in order to obtain a composite material part presenting        residual porosity of less than 11% by volume.

The term “thick” part is used herein to mean a part possessing thicknessof at least 5 centimeters (cm).

The fiber structure may be of the one-dimensional type (1D), e.g. formedby winding a yarn or a tow, of the two-dimensional type (2D), e.g.formed by draping fiber plies, or of the three-dimensional type (3D),e.g. formed by 3D weaving, braiding, or knitting, or by superposingfiber plies and bonding them to one another.

Fiber plies can be bonded to one another “mechanically” by means ofelements extending through the plies. This can be done by needling withfibers being moved out from the planes of the plies, or by implantingyarns or rigid elements (needles or rods) through the plies, or else bystitching. The fiber structure then constitutes a preform for the partthat is to be made, which preform can be manipulated while it conservesits cohesion, however it is not rigid.

In a variant, fiber plies constituting a 3D fiber structure can bebonded to one another by means of a bonding agent such as an organic oran inorganic binder that serves not only to bond the plies together, butalso to stiffen the fiber structure.

It should be observed that with 3D fiber structures constitutingnon-rigid fiber preforms, the preforms can be made rigid by beingconsolidated by being partially densified.

With a non-rigid fiber structure (a non-rigid 1D, 2D, or 3D structure),it is advantageous to compact the fiber structure. Compacting can beperformed at least in part by means of the flexible membrane duringpolymerization under pressure.

With a rigid fiber structure, a drain is advantageously placed betweenthe fiber structure and the flexible membrane, and the resin containedin the drain is forced to penetrate into the fiber structure duringfinal polymerization under pressure.

Either way, by the associated presence of a flexible membrane as a wallof the mold and of polymerization under pressure, the porosity of thecomposite material is thus reduced.

It is possible to use a mold having a rigid support part with a surfacecorresponding to the profile of one of the surfaces of the part that isto be made and against which the fiber structure is applied.

According to a particular feature of the invention, it is possible toperform pre-distillation treatment on the resin composition before it isinjected into the mold in order to reduce the volatile material contenttherein to a value of less than 25% by weight.

The resin used is a polycondensation resin, such as a phenolic resin, inparticular of the resol type, or a furanic resin. Solid fillers infinely-divided form may be added to the resin.

Advantageously, the polymerization step includes a initial stage duringwhich the temperature is raised to a first value and suction isestablished within the mold in order to evacuate the volatile materialsthat are produced, and a final stage during which the temperature israised progressively from the first value and the pressure is raisedinside the enclosure in order to apply to the impregnated fiberstructure a pressure that is preferably greater than 1 megapascal (MPa),e.g. lying in the range 1 MPa to 2.5 MPa.

Thus, the method of the invention is remarkable in that it constitutesadapting the RTM method specifically for making parts that are thick andof low porosity.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be better understood on reading the followingdescription given with reference to the accompanying drawings, in which:

FIG. 1 shows the successive steps in an implementation of a method inaccordance with the invention;

FIG. 2 is a highly diagrammatic overall view of an installation enablinga method of the invention to be implemented;

FIGS. 3 and 4 are diagrammatic views of embodiments of a mold for makinga diverging portion of a rocket engine nozzle from a non-rigid fiberpreform;

FIG. 5 is a diagrammatic view of an embodiment of a mold for making adiverging portion of a rocket engine nozzle from a rigid, consolidatedfiber preform; and

FIG. 6 shows how pressure and temperature vary during the step ofpolymerizing the resin in a particular implementation of the method inaccordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A first step 10 of the method consists in preparing a fiber structurethat is to constitute the reinforcement of a composite material partthat is to be made. The fiber structure may be in the form of athree-dimensional (3D) fiber preform of a shape that corresponds to theshape of the part that is to be made, and itself presenting sufficientcohesion to enable it to be handled without losing its cohesion. Inwell-known manner, such a 3D fiber preform may be a felt or it may beobtained by three-dimensional weaving, knitting, or braiding, or bysuperposing two-dimensional (2D) plies and bonding them together. The 2Dplies may be in the form of woven fabric, or of unidirectional (UD)sheets, or multidirectional sheets made up of a plurality of UD sheetssuperposed in different directions and bonded to one another. 2D pliescan be bonded together by needling, stitching, or indeed inserting rigidelements or yarns through the plies.

By way of example, reference can be made to U.S. Pat. Nos. 4,790,052 and5,226,217 which describe making 3D fiber structures of various possibleshapes.

It is also possible to use a 3D stiffened fiber preform suitable forbeing manipulated while conserving its shape with the help of supportingtooling. Such a preform can be obtained by consolidating a non-rigid 3Dfiber preform by depositing within the fiber preform material in aquantity that is just sufficient to bond the fibers of the preform toone another, i.e. by pre-densification of the preform. This can beperformed by chemical vapor infiltration (CVI) or by using a liquidtechnique, i.e. by impregnating the preform with a liquid precursor ofthe consolidation material, e.g. a resin, and transforming the precursorby heat treatment, while maintaining the shape of the fiber preform bytooling.

A rigid 3D fiber preform can also be obtained by superposing 2D plies,with the plies being bonded to one another by a binder that is organic(resin) or inorganic.

As mentioned above, it is also possible to use 1D fiber structures, e.g.obtained by merely winding a yarn, a tow, or a ribbon, or 2D fiberstructures, e.g. obtained merely by draping 2D plies.

A second step 11 of the method consists in placing the fiber structurein a mold of a densification installation, e.g. of the kind shown inFIG. 2.

The installation comprises an enclosure 20 forming an autoclave havingthe mold 22 containing the fiber structure placed therein on a tray 23.Embodiments of the mold 22 are described below with reference to FIGS. 2and 3.

A resin composition is introduced into the mold from an injection system24 and a pipe 25 connecting the injection system to the mold 22, e.g. atthe base of the mold. The injection system 24 comprises a tank and meansfor optionally heating and pressurizing the resin.

A pipe 26 connects the mold 22, e.g. the top portion thereof, to avacuum source (not shown). A valve 27 mounted in the pipe 26 serves toestablish or interrupt suction in the mold 12.

In addition, a pipe 28 connects the autoclave 20 to a source of gasunder pressure, e.g. nitrogen (not shown). A valve andpressure-regulator assembly 29 is mounted in the pipe 28 to enable ahigher pressure of desired value to be established or interrupted withinthe autoclave 20.

The autoclave 20 is also conventionally provided with heater means, e.g.of the resistive type, associated with temperature regulator means (notshown).

In the installation of FIG. 2, the resin is introduced via the base ofthe mold and rises progressively under the action of the suctionestablished inside the mold by the connection to the vacuum source,possibly assisted by the resin being delivered at a higher pressure.Naturally, other dispositions are possible, by reversing the directionof flow of the resin through the mold or by injecting the resin intovarious different levels of the mold, in particular when the part thatis to be made is of large dimensions.

Molds that are suitable for annular fiber structures are shown in FIGS.3 to 5, in an example relating to making a diverging portion for arocket engine nozzle. The fiber structure is then preferably a 3Dpreform of carbon fibers made up of (woven or sheet) fiber plies thatare superposed and bonded to one another by needling. The preformpresents a shape that is circularly symmetrical and annular,frustoconical, or curved in profile, like an egg cup. Its dimensionsvary depending on the dimensions of the diverging portion that is to bemade. The thickness of the preform exceeds 5 cm and may be as great as15 cm or even more.

Naturally, the shapes of the fiber structures and of the mold need to beadapted on each occasion to the shapes of the parts that are to be made.

The molds of FIGS. 3 and 4 are suitable for a fiber preform that has notbeen pre-densified, and that is therefore not rigid, also known as a“dry” preform.

The mold of FIG. 3 comprises a part or support tooling 32 having a base32 a resting on a support 33 and projecting in the form of a core 32 bhaving an outside surface that is substantially frustoconical and ofprofile that corresponds to the profile of the surface that is to definethe gas flow stream within the diverging portion of the nozzle that isto be made.

The preform 30 of the diverging portion that is to be made issubstantially annular in shape and is placed on the core 32 b with oneaxial end resting on the base 32 a.

A resin diffusion drain 34 is placed on the outside surface of thepreform 30, with the drain 34 being in the form of a grid, for example.The assembly is covered by a flexible leakproof membrane 36 made ofelastomer, e.g. of silicone. The cover 36 is secured in leaktight mannerby clamping collars around the base 32 a and around an extension 32 c atthe top of the core 32 b.

The pipes 25 and 26 are connected in leaktight manner to openings formedin the membrane 36 respectively level with the bottom and top portionsof the core 32 a.

A resin diffusion channel 38 extends around the bottom portion of thepreform 30 and is fed with the resin by the pipe 25. By way of example,the channel 38 is constituted by a pierced tube.

The resin introduced via the pipe 25 spreads around the bottom portionof the preform in the channel 38 and progresses along the diffusiondrain 34 so as to penetrate into the preform 30 from the drain. Excessresin is taken by the evacuation pipe 26 that is connected to an orificeformed in the membrane 36 at the top portion of the tooling 32.

FIG. 4 shows a mold comprising rigid support tooling 42 of female shape,unlike the tooling of FIG. 3. This tooling comprises a frustoconicalportion 42 b that is closed at its smaller-diameter top end 42 c andthat is provided with a collar 42 a around its open bottom end. Theinside face of the tooling corresponds to the profile that is desiredfor the outside surface of a diverging portion that is to be made.

The preform 40 of the diverging portion is placed against the insidesurface of the frustoconical portion 42 b of the tooling. A resindiffusion drain 44 is placed against the inside surface of the preform40, the drain being in the form of a grid, for example. A flexibleleakproof membrane 46 covers the drain 44, the membrane being made ofelastomer, e.g. of silicone. The membrane extends continuously over theentire inside surface of the assembly formed by the tooling 42. At itsperiphery, it is held clamped in leaktight manner against the collar 42a and against a support 43. In its central portion, it is held clampedin leaktight manner between the top 42 c of the tooling and a backingpiece 47.

A resin diffusion channel 48 extends around the bottom portion of thepreform 40 and is fed from the inside by the pipe 25. By way of example,the channel 48 is formed by a pierced tube.

The resin introduced by the pipe 25 spreads around the bottom portion ofthe preform, travels along the diffusion drain 44, and penetrates intothe preform 40 through the drain. Excess resin is taken by theevacuation pipe 26 that is connected to an orifice formed in the top 42c of the tooling.

A mold suitable for a consolidated rigid 3D preform is shown in FIG. 5.The preform may be consolidated by depositing pyrolytic carbon (PyC) byCVI so as to bond the fibers to one another, the CVI deposition processand PyC themselves being well known.

The consolidated fiber preform 50 is enclosed in leaktight mannerbetween inner and outer membranes 52 and 54, and it stands on a support53. Leakproof elastomer membranes are used, e.g. made of silicone, and adelamination ply 55 and a drain fabric 56 are interposed between thepreform 50 and at least the outer membrane 54.

As shown in FIG. 5, the membranes 52 and 54 are pressed against eachother in leaktight manner at the support 53, and a rod 57 carried by thetray holds the inner membrane 52 pressed against the inside surface ofthe preform 50.

A resin diffusion channel 58 is formed around the bottom portion of thepreform 50, under the membrane 54 and is connected in leaktight mannerto the pipe 25. The resin introduced via the pipe 25 penetrates into thepreform through the drain fabric 56 and the delamination ply 55. Excessresin is retained by the fabric 56 and by the pipe 26. The delaminationply 55 serves to facilitate unmolding after the resin has polymerized.

After the fiber structure has been put into place in the enclosure 20,the following step 12 consists in preparing the resin for injection intothe mold 22.

The method of the invention is particularly suitable for makingcomposite material parts having a matrix of a polycondensation resin, inparticular a phenolic polycondensation resin. In particular, it ispossible to use a phenolic resin of the resol type. The phenolic resinsusually used in RTM processes have low viscosity. They are alsodelivered with a large amount of solvent as can be seen from theirvolatile material content, which content is relatively large, commonlyabout 40% by weight. During polymerization, the volatile materialgenerates considerable porosity, about 15%.

Thus, according to a feature of the invention, a resin is injected thathas a relatively low volatile material content, less than 25% by weight,and preferably less than 20%. The term volatile material is used hereinto mean the solvent associated with the resin and the other materialsthat are exhausted in gaseous form during the polymerization cycle.

Depending on the volatile material content present in the availableresin composition, it might be necessary to perform a pre-treatmentoperation in order to lower said content. Such pre-treatment consists invacuum pre-distillation, while maintaining the resin at a moderatetemperature. The temperature is selected to be high enough to achievepre-distillation, but without triggering polymerization of the resinsince that would impede its injection into the fiber preform that is tobe densified. With phenolic resins, in particular of the resol type, thetemperature is selected to lie in the range 60° C. to 90° C., forexample.

In order to impart viscosity to the resin composition that issufficiently low to enable thick fiber preforms for densification to beimpregnated right through to the core, it might be necessary to heat theresin so as to raise its temperature up to a required level in order toachieve the desired viscosity. In general, the viscosity should lie inthe range 0.1 Pa·s to 0.3 Pa·s, and should preferably lie in the range0.1 Pa·s to 0.15 Pa·s. With phenolic resins of the resol type having avolatile material content of less than 25%, the temperature shouldpreferably lie in the range 65° C. to 85° C., it being understood thatthe temperature must not exceed a threshold beyond which increasingviscosity of the resin prevents injection taking place.

The resin having the desired volatile material content and viscosity isinjected into the mold 22 (step 13) using the injection system 24, whichis adapted to deliver the resin composition at the desired temperatureand possibly under pressure, e.g. at a pressure potentially up to 3kilopascals (kPa). Simultaneously, the inside volume of the mold isevacuated by opening the valve 27. A counter pressure can be establishedin the autoclave 20 via the pipe 28 to balance pressure between theinside and the outside of the mold and to avoid the or each membranethereof ballooning.

After the resin has been injected, a step of polymerization within themold 22 is performed (step 14). FIG. 6 shows an example of temperatureand pressure variations within the mold and the autoclave during thepolymerization cycle. Advantageously, the polymerization step comprisesan initial stage 14 a during which the temperature T in the autoclave 20is raised to a level T₁ where it is maintained, and the mold 22 ismaintained under a vacuum by opening the valve 27, the pressure P₁ inthe autoclave 20 possibly being maintained equal to the surroundingpressure or equal to the counter pressure that is established duringinjection. The temperature is raised to a value T₁ that is sufficient toencourage degassing from the resin, i.e. evacuating the volatilematerial contained in the resin, while keeping the resin sufficientlyfluid to enable it to continue to flow within the fiber preform and fillin the pores as the volatile material are evacuated via the pipe 26.With phenolic resins of the resol type, this temperature T₁ liespreferably in the range 65° C. to 85° C. The duration t₁ of the initialstage of vacuum degassing in the polymerization cycle is selected toenable sufficient volatile materials to be evacuated to obtain, afterpolymerization, a desired residual level of porosity in the resinmatrix. This duration t₁ may last for several hours or several tens ofhours.

Thereafter, a final stage 14 b of polymerization under pressure isperformed. To do this, the evacuation of the mold 22 is interrupted byswitching off the vacuum, and then the autoclave 20 is pressurized andthe temperature in the autoclave is raised progressively in steps up toa final polymerization temperature T_(f).

The pressure P₂ in the autoclave is raised to a value that is relativelyhigh, preferably greater than 1 MPa, e.g. lying in the range 1 MPa to2.5 MPa. Under the effect of the pressure, the resin composition creepsinto the residual pores within the fiber structure.

When the fiber shaft is not rigid, the pressure in the autoclavecompresses the fiber structure, thus making it possible to obtain acomposite material part that not only has small porosity, but that alsohas a fiber volume fraction that is increased relative to that to thefiber structure, the fiber volume fraction being the fraction of theapparent volume of the fiber structure or of the part that is occupiedby the fibers.

It should be observed that the non-rigid fiber structure could also becompacted or pre-compacted at a stage prior to that of polymerizationunder pressure.

With a rigid fiber structure, the pressure in the autoclave allows theresin composition stored in the drainage fabric of the mold to be causedto creep into the fiber structure, thereby reducing its porosity.

The temperature T_(f) depends on the type of resin composition that isused. Thus, with phenolic resins of the resol type, the temperatureT_(f) is preferably greater than 160° C.

Once polymerization is terminated, the heating of the autoclave isinterrupted and the pressure within the autoclave is returned toatmospheric pressure.

In order to make it easier to fill in the pores of the fiber structureso as to reduce the residual porosity of the composite material part, itis possible to use a resin composition that contains solid fillers. Thefillers must be in divided form and in limited quantity so as to avoidcompromising injection of the fiber structure to its core. Thus, thepercentage by weight of solid fillers should preferably be less than10%. By way of example, it is possible to use carbon black.

Although carbon fibers are described as being used for making the fiberstructure, it can readily be seen that fibers of some other kind couldbe used, such as organic or inorganic fibers, e.g. glass fibers orceramic fibers (silica, alumina, . . . ).

In addition, the use of a polycondensation resin other than a phenolicresin is possible providing the steps of preparing the resincomposition, injecting it into the mold, and polymerizing it can beimplemented in a manner similar to that described above. Thus, it ispossible to envisage using resins of the furanic type.

Tests have been carried out on dry 3D fiber preform samples made up ofcarbon fiber fabric plies superposed on a mandrel and bonded to oneanother by needling in order to make up fiber preforms of substantiallyfrustoconical shape possibly reaching an axial length of 110 cm and anoutside diameter of 200 cm.

A resol type phenolic resin composition was used that was pre-distilledto present a volatile material content of about 20% by weight. The resincomposition was injected at a temperature of about 85° C. under apressure of 0.2 MPa, with a vacuum being established within the mold.

The polymerization cycle comprises an initial phase of duration t₁ at atemperature in the range 65° C. to 85° C. with the mold being evacuatedand without raising pressure in the autoclave, and a final stage under apressure lying in the range 1 MPa to 2.5 MPa in the autoclave withtemperature being raised in steps up to about 160° C.

For values of t₁ going from several hours to several tens of hours, thefollowing were observed on the various resulting densified parts:

-   -   variation in relative density lying in the range 1.35 to 1.43;    -   variation in residual open porosity lying in the range 5.9% to        10%;    -   variation in the compacting ratio lying in the range 8% to 37%,        where compacting ratio is the relative decrease in volume        between the part as obtained and the fiber preform; and    -   variation in fiber fraction lying in the range 38% to 54%.

These tests confirm that it is possible using a method of the inventionto densify thick fiber textures using an RTM type method while obtainingresidual porosity that is small, less than 11%, and with it beingpossible, when using “dry” fiber textures, to achieve a fiber fractionthat is quite high.

It should also be observed that by selecting a long duration t₁, i.e. aduration of several tens of hours, it is possible to obtain residualporosity that is small but that reduces the capacity for compacting andincreasing the fiber fraction because of the greater viscosity of theresin at the end of the initial stage of the polymerization step.

Naturally, parts of dimensions larger than those of the parts madeduring the above-described tests can be fabricated.

1. A method of making a thick part out of composite material comprisingfiber reinforcement and a resin matrix, the method comprising the stepsof: providing a fiber structure that is to form the reinforcement of thepart to be made; placing the fiber structure in a mold having at leastone wall that is formed by a flexible membrane; injecting into the molda resin composition having a volatile material content of less than 25%by weight and at a temperature of a value such that its viscosity liesin the range 0.1 Pa·s to 0.3 Pa·s; and polymerizing the resin in themold placed in an enclosure with temperature being raised progressively,the polymerization step including at least a final stage ofpolymerization under pressure in order to obtain a composite materialpart presenting residual porosity of less than 11% by volume.
 2. Amethod according to claim 1, in which a fiber structure is usedcomprising two-dimensional plies that are superposed and bonded to oneanother.
 3. A method according to claim 2, in which the two-dimensionalfiber plies are bonded to one another by elements extending through theplies.
 4. A method according to claim 2, in which the two-dimensionalfiber plies are bonded to one another by an organic or an inorganicbinder.
 5. A method according to claim 1, in which a non-rigid fiberstructure is used and the fiber structure is compacted.
 6. A methodaccording to claim 5, characterized in that the compacting is performedat least in part via the flexible membrane during the finalpolymerization under pressure.
 7. A method according to claim 1, inwhich a rigid fiber structure is used, a drain is placed between thefiber structure and the flexible membrane, and the resin contained inthe drain is forced to penetrate into the fiber structure during thefinal polymerization under pressure.
 8. A method according to claim 7,in which a fiber structure is used that is stiffened by consolidation bypartial densification.
 9. A method according to claim 1, in which a moldis used comprising a rigid support portion having a surfacecorresponding to the profile of a surface of the part to be made andagainst which the fiber structure is applied.
 10. A method according toclaim 1, in which a pre-distillation treatment is performed on the resincomposition before it is injected into the mold in order to reduce thevolatile material content thereof to a value of less than 25% by weight.11. A method according to claim 1, in which a resin is used that isselected from phenolic resins and furanic resins.
 12. A method accordingto claim 1, in which the resin composition also contains solid fillers.13. A method according to claim 1, in which the polymerization stepincludes an initial stage during which the temperature is raised to afirst value and suction is established in the mold in order to evacuatethe volatile materials that are produced, and a final stage during whichthe temperature is raised progressively from the first value, and thepressure is raised in the enclosure in order to apply a pressure lyingin the range 1 MPa to 2.5 MPa on the fiber structure.
 14. A methodaccording to claim 2 in which: the two-dimensional fiber plies arebonded to one another by an organic or an inorganic binder; a rigidfiber structure is used, a drain is placed between the fiber structureand the flexible membrane, and the resin contained in the drain isforced to penetrate into the fiber structure during the finalpolymerization under pressure; a fiber structure is used that isstiffened by consolidation by partial densification; a mold is usedcomprising a rigid support portion having a surface corresponding to theprofile of a surface of the part to be made and against which the fiberstructure is applied; a pre-distillation treatment is performed on theresin composition before it is injected into the mold in order to reducethe volatile material content thereof to a value of less than 25% byweight; a resin is used that is selected from phenolic resins andfuranic resins; the resin composition also contains solid fillers; thepolymerization step includes an initial stage during which thetemperature is raised to a first value and suction is established in themold in order to evacuate the volatile materials that are produced, anda final stage during which the temperature is raised progressively fromthe first value, and the pressure is raised in the enclosure in order toapply a pressure lying in the range 1 MPa to 2.5 MPa on the fiberstructure.
 15. A method according to claim 3 in which: a rigid fiberstructure is used, a drain is placed between the fiber structure and theflexible membrane, and the resin contained in the drain is forced topenetrate into the fiber structure during the final polymerization underpressure; a fiber structure is used that is stiffened by consolidationby partial densification; a mold is used comprising a rigid supportportion having a surface corresponding to the profile of a surface ofthe part to be made and against which the fiber structure is applied; apre-distillation treatment is performed on the resin composition beforeit is injected into the mold in order to reduce the volatile materialcontent thereof to a value of less than 25% by weight; a resin is usedthat is selected from phenolic resins and furanic resins; the resincomposition also contains solid fillers; the polymerization stepincludes an initial stage during which the temperature is raised to afirst value and suction is established in the mold in order to evacuatethe volatile materials that are produced, and a final stage during whichthe temperature is raised progressively from the first value, and thepressure is raised in the enclosure in order to apply a pressure lyingin the range 1 MPa to 2.5 MPa on the fiber structure.